The present invention relates to an anti-lock control system for motor vehicles including wheel sensors, a monitoring circuit which recognizes overbraking conditions of the wheels based on the signals from the sensors and produces corresponding control signals, and a pressure modulator which, by means of the control signals, sets the brake pressures on the vehicle wheels.
ABS control systems switch cyclically between the typical "decreasing pressure," "maintaining pressure," and "pressure build-up" phases. Knowledge of pressure reference points, in addition to the determination of suitable switching times, is of decisive importance in order to obtain large uniform braking decelerations of the vehicle with the greatest possible comfort.
If one assumes uniform conditions of the pavement and furthermore assumes that the properties of the tires and brakes do not change substantially during braking, an anti-lock system could achieve optimal braking by establishing a brake pressure which lies a differential amount below the locking pressure on each wheel that is in danger of locking. This pressure could be stored for each wheel, the first time a tendency toward locking occurs while braking. Since this optimal pressure, which has been determined upon counteracting the tendency towards locking, is below the locking pressure, the braking could then be carried out to the end with maximum deceleration, good travel stability and high control comfort. Should a wheel nevertheless tend to lock after the optimal pressures have been established in this way, the adjustment process would be repeated on basis of a newly determined locking pressure.
This simple strategy, however, cannot be employed in practice, since wheel locking pressures may be subject to substantial variations for the following reasons:
Although practically all actual braking takes place with more or less homogeneous conditions of the pavement, enormous local variations in coefficient of friction nevertheless can occur. PA1 The dynamic behavior of the vehicle leads to changes in the wheel contact forces (in particular, changes in load caused by curves or by vehicle pitching, caused in turn by changes in deceleration during the initial braking phase). PA1 During braking from high initial speed, the characteristics of the tires may change greatly due to heating. PA1 The brake system may exhibit fading effects during a lengthy braking, so that setting a constant brake pressure leads to a brake force which decreases with time.
It therefore may be necessary, in the extreme case, to further increase the brake pressure on one or more wheels which traveling in a stable wheel slippage region, in order to again obtain the main brake cylinder pressure desired by the-driver and thereby obtain dependable controlled braking.
Furthermore, after a tendency towards locking has just been counteracted, it is not permissible for the pressure to return as rapidly as might be desired into the vicinity of the former locking pressure, since changes in the wheel contact forces caused by the spring suspension can lead to dynamic changes in the locking pressure.